1. Field of the Invention
The present invention belongs to a technical field relating to an apparatus and method for producing solid fuel using low-grade coal as a raw material.
2. Description of the Related Art
With respect to a technique for producing solid fuel using low-grade coal as a raw material, a known method for producing solid fuel is that disclosed in Japanese Unexamined Patent Application Publication No. 7-233383. The method for producing solid fuel disclosed in this document comprises mixing an oil mixture containing heavy oil and solvent oil with porous coal to prepare a raw material slurry, heating the slurry to progress dehydration and fill the pores of the porous coal with the oil mixture containing the heavy oil and the solvent oil, and then subjecting the slurry to liquid/solid separation. The porous carbon corresponds to low-grade coal.
The method for producing solid fuel disclosed in the above document can produce dehydrated solid fuel having increased calorie value, low spontaneous combustibility, and excellent transport and storage quality.
Namely, the porous coal (low-grade coal) contains a large quantity of water, and thus transport of the porous coal requires transport of water, thereby increasing transport cost. Therefore, the transport quality is degraded, and the calorie value is decreased due to the high water content. Therefore, it is desired to dehydrate the porous coal. However, the dehydration by a usual drying method has the danger that a spontaneous combustion accident occurs due to oxygen adsorption and oxidation reaction at active sites present in the pores of the dehydrated porous coal.
On the other hand, in the method for producing solid fuel disclosed in the above-described document, the raw material slurry (mixture containing the porous coal and the mixed oil containing the heavy oil and the solvent oil) is heated to evaporate the moisture in the pores of the porous coal and coat the pores with the oil mixture containing the heavy oil and finally fill the pores with the oil mixture, preferentially the heavy oil. As a result, the oxygen adsorption and oxidation reaction at the active sites present in the pores are suppressed, thereby suppressing spontaneous combustion. Furthermore, the coal is dehydrated by heating, and the calorie thereof is increased by the dehydration and oil filling in the pores, thereby producing dehydrated solid fuel having increased calorie value, low spontaneous combustibility and excellent transport and storage quality.
With respect to an apparatus for producing the above-mentioned solid fuel, the above document discloses an apparatus for producing solid fuel comprising a mixing tank for mixing the oil mixture containing the heavy oil and the solvent oil with the porous coal to prepare the raw material slurry, an evaporator for removing water vapor by heating the raw material slurry, and a liquid/solid separator for liquid/solid separation of the heated slurry. The porous coal corresponds to low-grade coal.
The apparatus for producing solid fuel disclosed in the above-described document is a basic apparatus for producing the above-described solid fuel. When the apparatus is used, in order to maintain a slurry state in the mixing tank and the evaporator, the slurry is stirred with a stirrer, and, at the same time, the slurry is circulated by a slurry pump to cause a turbulent state for preventing coal deposition and maintaining the slurry state. The slurry is circulated through a slurry circulating passage having the slurry pump provided therein. In the mixing tank, the slurry is circulated by introducing the slurry to the top of the mixing tank from the bottom of the mixing tank through the slurry circulating passage thereof. In the evaporator, the slurry is circulated by introducing the slurry to the top of the evaporator from the bottom of the evaporator through the slurry circulating passage thereof.
The slurry state is maintained by stirring and circulating the slurry, but when the stirring and circulation of the slurry are stopped by a power failure, an apparatus failure, or the like, the solid (coal) contained in the slurry settles to cause blocking due to coal deposits at the bottom of the mixing tank, a portion (for example, a portion between the bottom of the mixing tank and the slurry pump) of the slurry circulating passage of the mixing tank, the bottom of the evaporator, a portion (for example, a portion between the evaporator and the slurry pump) of the slurry circulating passage of the evaporator, and the like.
Due to this blocking, the apparatus cannot be restarted. In order to permit restarting, therefore, the apparatus is disassembled (for example, the pipes constituting the slurry circulating passage are removed) to remove the slurry, and the insides of the pipes, the mixing tank, and the evaporator are washed with oil.
In this case, the slurry in the evaporator contains moisture at high temperature and high pressure, and thus the slurry escapes when the vessel (evaporator) is opened, thereby requiring clean up. Therefore, the slurry is usually allowed to cool to a temperature close to room temperature. However, the cooling requires several days because the evaporator is covered with a heat insulator, and thus the operation must be stopped for a long period of time, thereby requiring a long time for restarting the operation.